The present invention relates to class D amplifiers, and in particular, to circuits and methods for reducing pop noise in class D amplifiers.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A switching amplifier, sometimes referred to as a class D amplifier, is an amplifier where the output transistors are operated as switches. One example of a transistor used in switching amplifiers is a MOSFET. When the transistor is off, the circuit behaves like an open circuit so the current is zero. When the transistor is on, the voltage across the transistor is ideally zero. In practice, the voltage is very small. Since the equation for power is P=V*I, the power dissipated by the amplifier is very low in both states. This increases the efficiency, thus requiring less power from the power supply and allowing smaller heat sinks for the amplifier, for example. The increased efficiency translates into benefits such as longer battery life. The decrease in the size of the heat sinks lowers the weight, cost, and size of the amplifier. Example applications where these advantages would be useful are portable battery-powered equipment such as cellular technology or portable music players.
FIG. 1 illustrates a signal received by a switching amplifier. A continuous input signal is received by a modulator 101 and converted into a train of pulses. The input signal is transformed into a stream of pulses where the pulse characteristics are linked to the amplitude of the input signal. For example, within each period, the duty cycle of a pulse may be proportional to the amplitude of the input signal. For instance, if the input signal received is constant at zero, the duty cycle of the output pulses may be 50%. If the input signal received is highly positive, the duty cycle of the output pulses may be near 100%. Conversely, if the input signal received is highly negative, the duty cycle may be near 0%.
The modulated signal is then amplified in a switching output stage 102. Since the modulated signal is represented by a train of pulses, the output transistors operate like switches. This enables the transistors to have zero current when they are not switching and a low voltage drop across the transistors when they are switching.
The amplified signal generated by output stage 102 then enters a filter 103 before entering a speaker 104. The filter translates the modified amplified signal back into a continuous signal. A typical filter is an LC filter, for example. The resulting amplified continuous signal may be provided to a speaker and translated into sound. The benefits of filters include minimizing electromagnetic interference (“EMI”) and power dissipation in the amplified signal.
However, one disadvantage of switching amplifiers in audio applications is the clicking and popping sounds that may occur during power up, power down, or other abrupt changes in the operating conditions of the class D amplifier. For example, filter 103 may include a capacitor which forms a high pass filter with an impedance of speaker 104. The low pass portion of filter 103 may allow this output signal to be coupled to speaker 104 through this capacitor and sound like clicking, popping, or other switching sounds from speaker 104.
Some systems have utilized muting and enable circuits to reduce switching sounds caused by class D audio amplifiers. Other systems sense when the system is powering up and provide alternate signals for a period of time before the audio is reconnected to the driving circuitry. Current solutions like these are complicated, cause an interruption in the audio signal channel, and may provide delays to startup.
Thus, there is a need for improved class D audio amplifiers. The present invention solves these and other problems by providing circuits and methods for reducing pop noise in class D amplifiers.